Method of chromium plating



c. v. IREDELL mwm METHOD OF QHROMIUM PLATING Filed Dec. 11, 1928 III]: III] INVENTOR CM IREDELL A'i'TOR Patented Apr. 3, 1934 METHOD OF CHROMIUM PLATING Charles V. Iredell, East range,N. J., assignor to Westinghouse Lamp Company, a corporation of Pennsylvania Application December 11, 1928, Serial No. 325,280

2 Claims.

This invention relates to the art of electrochemistry and more particularly relates to the art of electro-plating metals from aqueous solutions and comprises essentially in a method of electroplating chromium upon metal filament, wires and the like.

In the electroplating of chromium the prior art has been chiefly concerned with the obtainance of a hard, dense, brittle coating of chromium for the purpose of furnishing a brilliant reflecting surface, or a wear resisting and tarnish resisting coating. When such a hard dense brittle coating is applied to base metal of relatively small dimensional measurement such as wire,

filament and the like, it is often found that the composite is brittle and fractures readily or that such a chromium coating strips flakes or chips off when the composite article is wound upon a spool, or bent as by forming into coils, V or W shaped articles.

In the incandescent lamp art it is desirable to utilize a coating of chromium upon metal filaments and wires of various sorts, such as tungsten, molybdenum, nickel, nickel-steel, copper clad nickel steel, etc. For most of these uses the base metal is of relatively small cross sectional dimension such as from 5 to mil (.005 to .010 inches) diameter. With filaments and wires of such diameter brittle coatings of electrodeposited chromium are highly undesirable.

It is one of the objects of this invention to provide an electrolytic method ofelectro-depositing chromium upon metal surfaces in a relatively pliable adherent state, substantially resistant to stripping and flaking from said metal base when the composite is subsequently mechanically deformed as by bending.

Another object of this invention is to provide a method of coating metal wires and filament and the like with an adherent, relatively pliable electrolytically deposited coating of chromium which is substantially resistant to stripping and flaking from the metal base when the composite is subsequently mechanically deformed as by bending.

coating of chromium to conform with the above objects.

Other objects and advantages will be apparent as the invention is more fully disclosed.

In accordance with these objectives I have determined that the hard brittle structure usually obtained in the plating of chromium is primarily the result of the occlusion or absorption within the plate of colloidal trivalent chromium compounds. The colloidal trivalent chromium compounds, commonly called chromium chmmates are produced by the combined action of the reducing eliect of the hydrogen liberaed at the cathode and of various impurities present in C5 solution or introduced in the solution during the plating operations, which tend to augment or increase this reducing action at the cathode and formation of colloidal chromium chremate.

In the electrolytic deposition of ob omium there are three types of baths or solutions recommended (see Bureau of Standards Bulletin #346) which are (1) acid, (2) neutral and (3) basic, wherein the components are respectively (1) chromic acid and sulfuric acid (2) chromic acid Z5 and chromic sulfate and (3) chromic acid, chromic sulfate and chromium chromate.

As indicated in the above reference, the electro-chemical properties of these three solutions are considered to be identical and upon electrolysis are believed ,to yield the same results from a plating standpoint.

As above noted, the prior art has desired this hard brittle coating in the various commercial applications for which chromium plated surfaces have been developed. For the purposes of this invention I have determined that I must eliminate those factors from the deposition process which cause embrittlement of the deposited metal.

I have determined that the embrittlement of the chromium coating caused by the trivalent chromium may be materially reduced by lowering the temperature of the plating solution during deposition and by limiting the extent of or amount of the basicity of the bath or solution.

The solution reaction as a result of the electrolytic decomposition of a chromic acid solution containing ionized sulfate compounds is substantially the liberation at the cathode of hydro- 10c gen and chromium and a liberation of the carrier $04 at the anode. There is a reducing effect upon the chromic acid solution promoted at the cathode due to the liberation of the hydrogen and an oxidizing effect promoted at the anode-brine to the liberationnf the S04. The reducing effect is normally greater than the oxidizing effect with the result that there is gradually built up in solution a content of trivalent chromium (Cr which is believed to react to form what is called chromium chromate. This compound is believed to be colloidal in character. The rate of formation of this colloid is accelerated by the presence in solution of various impurities which tend to augment the reducing action at the cathode, or to increase the basic nature of the solution.

It is well known in the art that electrolytically deposited chromium 'contains large quantities of absorbed or occluded hydrogen and it is commonly believed that this gas also materially increases the hardness of the coating. I- have found, however, that the occlusion of hydrogen alone does not materially effect a hardening of electro-deposited chromium but that the occlusion or retention of the colloid chromium chromate within the coating is responsible for the major portion of the hardness developed in electrolytically deposited chromium.

The amount of hydrogen evolution at the cathode is materially increased by the use of increased temperatures, and such increased hydrogen evolution increases the trivalent chromium content or rate of formation and materially decreases the current efficiency of the solution.

I have determined that the operating temperatures of the bath should be maintained relatively low, approximating 20 to 30 C. At these temperatures I am enabled to obtain current efiioiencies ranging from 40 to 170% as compared to the current efficiencies of from 12% to 18% heretofore obtained at temperatures of 40 C. and over, recommended in the prior art. This permits of a greater speed in coating than here tofore obtainable for the reason that I may employ at such temperatures a higher current density than heretofore permissible.

As above noted, the absorption of hydroge alone does not materially affect the hardness of the chromium deposit formed at this temperature from the bath and the formation of and accumulation in solution of the deleterious hardening colloid reduction product commonly spoken of as chromium chromate or trivalent chromium, is materially retarded or lessened.

Inasmuch as even under these conditions the reduction or formation of this colloid at the cathode is always greater in amount than can be oxidized at the anode, this colloid accumulates in solution on continued use until it reaches a proportion sufficient to materially effect a decrease in the current efficiency with a consequent increase in hydrogen evolution and a consequent greater rate of formation of the colloid. When this point is reached I may'not utilize the solution further for the purpose of my invention as the deposited chromium then is sufficiently embrittled to cause the coating to chip or shatter from the surface of the wire on bending, spooling, etc. The brittle structure may become sufiicient to cause some fracture in the base metal on bending.

At this point I must either discard the bath or substantially effect the precipitation of the colloidal trivalent chromium (chromium chromate) content of the solution as an insoluble compound before the solution may be further employed. Due to the colloidal nature of the trivalent chromium or chromium chromate as it is called, this precipitation may not be so readily or quickly made from solution by the addition thereto of precipitating reagents. I have found, however, that a precipitation of the trivalent chromium may be effected by leaving the solution stand in contact with sheet lead for an interval of time whereby the lead substantially effects a substitution or reaction with the chromium in the colloid chromium chromate or trivalent chromium compound and the chromate radical thereof is deposited as an insoluble lead chromate. The solution may then be again utilized in the plating operation, it being appreciated that the chromic acid content should be replenished from time to time to maintain the same to approximately the same amount as initially present.

The rejuvenated or trivalent chromium free solution may be again electrolyzed until an amount of trivalent chromium is again built up in the solution which will cause embrittling of the electrolytically deposited metal.

It is apparent, therefore, that for the purpose of my invention that I am limited to the use of a plating solution which may be either acidic or neutral in character and which may contain only a. limited amount of constituents therein tending to establish a solution which is basic in character as an appreciable basic content to the solu- -tion causes the embrittlement of the deposited chromium.

I have determined that compounds of those elements which would tend to form compounds of a basic character at the cathode during electrolysis should particularly be eliminated from the plating solution and prevented from entering therein during the plating operation. Such elements may be designated as being the alkaline and alkaline earth metals and more particularly the alkali metals.

The exact reaction involved wherein the alkali metals react in the plating solution to form the chromium chromate colloid is not at present apparent. It is believed however, due to the fact that the alkali metals upon being liberated at the cathode, react to form alkali hydroxide and liberating thereby more hydrogen. The combined effect of the alkaline reaction of the hydroxide and the increased reducing action of the added amount of hydrogen accelerates the formation of the colloidal chromium chromate and consequent embrittlement of the deposited plate. It appears that very small amounts of ionized alkali metals are required to produce this embrittling effect.

I therefore provide in the solution in addition to the usual chromic acid and sulfate ions, of the so-called Sargent solution, a proportion of an acid radical which is capable of reacting with the alkali metal ion to give an insoluble or very difficultly soluble compound. It is also essential that the said acid radical is not of such a nature as to deleteriously enter into or effect the chromium plating in any manner. I have found that a proportion of fiuosilicic acid radical or tartaric acid radical is the most satisfactory for my purpose. The latter may only be used for the precipitation of potassium whereas the former may be utilized for the precipitation of either potassium or sodium.

In the plating of most metal wires or filaments it is customary to subject them to a caustic alkali wash at some stage prior to coating to remove the greases of wire drawing lubricant or that may be accumulated thereon incident to handling, storing, etc. The alkali metal compounds are frequently insufiiciently removed from the surface of such cleaned metals and are frequently subsequently introduced into the plating solution and gradually accumulate therein to an amount sufficient to cause the embrittlement of the resulting plate even though the initial solution or bath was substantially free of such alkali metals. As caustie soda is the alkali-most commonly employed in the caustic cleaning operation prior to plating, fiuosilicic acid is preferably employed as a neutralizing agent as this acid radical most efiiciently removes the sodium ion from solution as an insoluble compound.

In order to more clearly bring out the nature of the present invention, reference is had to the accompanying drawing in which is shown the apparatus I employ in the practice of my invention, wherein is shown the wire 1 which is to be plated coming from the unwinding head (not shown but indicated) passing over metal wheel contact 2 comprised of brass, copper, or other suitable material, which is connected to the negative terminal of the battery 3 over variable resistance 4 in parallel with metal wheel contacts 5 and 6. A suitable ammeter '7 is placed in the circuit as shown. Wire 1 passes into the electrolytic chamber 8 and is held under the surface of the bath by glass or porcelain wheels 9, 10, 11 and 12 by vertical supports in the manner shown. The electrolytic chamber 8 is preferably comprised of lead or lead lined iron and is connected electrically to the positive side of the battery 3. A suitable means of making and breaking the circuit is supplied as indicated by switch 13.

The wire 1 after passing a short distance (depending upon the length of the bath, filament size, current density employed, etc.) may be passed over the second cathode contact 5; if desired. The purpose of this contact is to equalize the plating process in a long bath employed to shorten the required time interval of plating. With heavier sized wires this contact may be dispensed with.

After leaving the plating bath over brass contact wheel 6 the Wire 1 passes over a hard rubber wheel 20 into a rinsing chamber 14 through which a continuous flow of pure water is maintained. The wire is held below the surface of the water by any suitable mechanical means such as by hard rubber wheels indicated at 15 and 16.

From the water bath the wire 1 is passed over hard rubber wheel 19 and then through any suitable drying means such as absorbent material 17 held in a clip through which the wire is drawn, and from thence to the winding head. The mechanical means of winding the wire and of maintaining a uniform rate of flow through the plating bath have not been shown as they constitute no integral part of the invention.

The entire apparatus is mounted in any suitable manner on a table 18. It is to be understood that the filament is treated as by scrubbing in hot alkali solution, annealing in hydrogen, and

in any other suitable means, tovremove from the surface the adherent coating of wire drawing lubricant prior to electroplating thereon the chromium coating.

' The specific plating solution I prefer to employ is of the so-called Sargents solution type and is comprised essentially of an aqueous solution of chromic acid (010:) to which has been added a sufiicient proportion of water soluble sulfate radical such as sulfuric acid, chromium sulfate,

etc, to accord with standard practice. An example of such a suitable bath is as follows:

Chromic acid (CrOa, anhydrous sulfate free) 250.0 gins. Concentrated sulfuric acid (H2804) 2.5-4.5 gms. Distilled water 1000 cc.

To this solution I add the desired amount of fiuosilicic acid preferably sufiicient to maintain a concentration of from 0.9 to 7 grams HzSiFs per liter; or I may add to the solution a proportion of an acid decomposable fiuosilicate compound which will yield a proportionate amount of the fiuosilicic acid radical ionized in solution. The acid tartrate radical may be introduced in a similar manner.

As a specific example of the practice of my invention I will describe the process I employ in chromium plating tungsten wire of .005 diameter in a continuous manner. v

With the particular apparatus used, the total length of the filament in the plating bath between the glass or porcelain wheels 9 and 10, 1-1 and 12 is 11.5 inches. The winding mechanism was adjusted to give a speed through the bath of approximately 60.5 inches per minute. The temperature of the plating solution was kept between 20 C. and 30 C. With this size filament (.005 inches) a current density of 12.7 amperes per square inch or a current of 2.3 amperes was applied, although I have found that higher current densities may be employed if desired resulting only in increased thickness in the deposited chromium. The upper permissible limit of current density under the above mentioned conditions of size of wire, rate through the solution and temperature of solution appears to be 33.0 amperes per square inch whereas the lower limit appears to be 8.5 amperes per square inch, as may be indicated in the following table:

In plating the tungsten wire for the specific purpose in view a weight approximating 1.0 mg.

per 200mm. length of wire is suitable and no specific advantage is obtained by forming heavier deposits even though the deposit is perfectly capable of being retained thereon after forming the coated. wire into the V, W or coil type filament desired.

Heavier deposits may also be obtained by increasing the length of the wire submerged, decreasing the speed or by repeating the plating operation to build up any required thickness of deposit.

I may calculate the maximum number of meters of plated filament I may obtain from any known volume of plating solution before the solution has reached the point where the brittle deposit is formed therefrom as aresult of the building up of the trivalent chromium content provided I maintain in the solution a sufiicient amount of lated life of the solution may be closely approximated. At the conclusion of this approximated time interval I remove the plating solution and replace with a fresh one and continue the plating operation.

I then regenerate or rejuvenate the old plating solution by permitting it to stand for an hour or so in contact with lead strips or store the same in lead lined jars and subsequently utilize the regenerated solution in my process. I have also found that ii". some cases I may add to the solution a proportion of solid potassium fiuosilicate which apparently augments or increases the rejuvenation of the plating solution in a manner not entirely understood at this time, but it is believed due to a physical effect produced by the potassium fiuosilicate crystals upon what may be described as a super-saturated sodium fiuosilicate solution. It is well known that sodium fluosilicate forms such superesaturated solutions and it is possible that the addition of solid potassium fiuosilicate crystals to the solution causes a precipitation of the excess sodium salt when left in contact therewith for a certain interval of time.

I may also utilize a continuous flow system in the plating bath by which the exhausted solution may be continuously passed out of the bath and caused to flow by gravity through an activating or regenerating chamber in which an intimate contact with lead is obtained and thence back into the electrolyzing chamber. At the present, such a continuously activating means is not essential to the successful application of my process but is contemplated as an improved feature thereof.

Having specifically directed this invention to the plating of tungsten filaments it is not to be construed that it is limited in scope thereby as I have successfully utilized the process in plating molybdenum, copper, nickel, nickel-iron alloys and copper clad nickel iron alloys, etc. As is well known, the current densities to employ in plating different metals and the resultant deposits obtained thereby will vary with the surface area and with the metal base being plated upon. Without specifically departing from the nature of the plating solution or from the temperature range (2030 C.) heretofore stressed and the time interval based upon the limits of 4.0 to 5.0 grams of plated chromium removal from solution prior to regenerating the solution, a firm, adherent, bright metallic coating may also be obtained on other metal bodies which is substantially capable of being subjected to severe mechanical deforming as by bending to shape at sharp angles, without fracture of coating or base metal or the stripping of the coating from the base metal, by varying the current densities to accord with the specific metalbeing plated upon and in accordance with the relative surface areas thereof.

For example, in chromium plating .005 inch molybdenum deposition of chromium is effected at current densities ranging from 2.6 to 21.4 amperes per square inch. The range wherein the best deposits may be obtained is from 5.0 to 13.0 amperes per square inch.

In chromium plating .010 inch copper clad nickel steel (Dumet wire) it is preferable to employ current densities between 1.6 to 14.0 amperes per square inch. Other metal filament bases will vary accordingly.

Having broadly outlined the scope of my invention and specifically directed the same to the production of a chromium coated tungsten filament which is substantially non-brittle and capable of being formed into desired shapes without fracture or stripping of the deposit, it is apparent that there are many variations and departures that may be made in the specific steps of my process without essentially departing from the nature of my invention as may be expressed in the following claims.

What is claimed is:

l. The method of electroplating a wire with a coating of chromium which will not flake upon deformation of said wire when plated comprising forming a solution of chromic acid (25%), sulfuric acid (2.5 to 4.5%) and electrolyzing said solution for only a limited interval of time substantially no greater than that required to deposit approximately 3 to 4 per cent of the original chromium content of the solution, the temperature of the solution being maintained between about 20 and 30 C. and the cathode current density being maintained between about 8.5 amperes per square inch and 33.0 amperes per square inch.

2. The method of electroplating a wire with a coating of chromium which will not flake upon deformation of said plated wire comprising forming a bath consisting of about 25% chromic acid, 2.5% to 4.5% sulfuric acid, .9 to 7% of fluosilicic acid and electrolyzing said solution for only a limited period of time, substantially no greater than that required to deposit approximately 3 to 4 percent of the original chromium content of the bath, the temperature of said bath being maintained between about 20 and 30 C. and he cathode current density being maintained between about 8.5 to 33 amperes per square inch. 

